Linked List Cycle II

Author: mengli

LinkedListCycleII.java

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+/**
+ * Given a linked list, return the node where the cycle begins. If there is no
+ * cycle, return null.
+ * 
+ * Follow up: Can you solve it without using extra space?
+ * 
+ */
+public class LinkedListCycleII {
+	public ListNode detectCycle(ListNode head) {
+		if (head == null)
+			return head;
+		ListNode fast = head, slow = head;
+		do {
+			if (fast.next != null && fast.next.next != null) {
+				fast = fast.next.next;
+				slow = slow.next;
+			} else {
+				return null;
+			}
+		} while (fast != slow);
+		slow = head;
+		while (fast != slow) {
+			fast = fast.next;
+			slow = slow.next;
+		}
+		return fast;
+	}
+}